Cathode ray tube with stainless steel grid sealed into glass envelope



Jan. 13, 1970 G. BRADU ETAL 3,489,966

CATHODE RAY TUBE WITH STAINLESS STEEL GRID SEALED INTO GLASS ENVELOPE Filed March 8, 1967 3 Sheets-Sheet l Geoeees 3M rem @Iemee Drawc m' Q ggg gem/P Jan. 13, 1970 BRADU ETAL 3,489,966

CATHODE .RAY TUBE WITH STAINLESS STEEL GRID SEALED INTO GLASS ENVELOPE Filed March 8, 1967 3 Sheets-Sheet 2 64 44M MQQM CATHODE RAY TUBE WITH STAINLESS STEEL GRID I SEALED INTO GLASS ENVELOPE Filed March 8, 1967 3 Sheets-Sheet 5 Jan, 13, 1970 BR ETAL 3,489,966

United States Patent Int. Cl. H01j 29/46 US. Cl. 313-83 4 Claims ABSTRACT OF THE DISCLOSURE Cathode ray tube comprising a glass envelope made up of two portions between which a post-focusing grid is sealed. The grid wires are cut at the outer surface of the envelope; a conducting layer coats the envelope zone which comprises the wire ends, and a layer of sealing material covers the conducting layer to ensure the airtightness between the two envelope portions.

The present invention relates to cathode ray tubes in which a grid of tensioned wires is provided for postacceleration, post-focusing, or as a barrier against secondary emission from adjacent electrodes.

The invention is of special interest, although not exclusively, in the case of television picture tubes having a focusing grid and a screen with luminescent strips, such as used in color television.

In these tubes the electron beam or beams, emitted from one or more electron guns, reach the luminescent strips of the screen after passage through one or more grids made up of stretched Wires and intended to focus correctly the electrons at the desired place by introducing both a post-acceleration and a post-focusing effect.

The grid or grids of these tubes must meet certain strict conditions for obtaining correct color pictures, which conditions generally complicate the manufacture of the tubes. These conditions are:

The grid must be a plane array of parallel metal wires, wherein the planeness of the assembly and the parallelism of the wires must be observed with high accuracy.

The tension of the wires must be as high as possible in order to reduce or suppress the wire vibration which would adversely affect the picture quality.

The mechanical tension which permits to fulfill both of the above conditions is fairly high and may be estimated at 700-800 grs. Since, on the other hand, the number of wires may be from. 250 to 600, or even more, depending on the size of the tube, the support in which the wire ends are inserted is submitted to a considerable stress and must therefore have a high degree of mechanical rigidity so as to resist deformation.

A glass envelope for the tube presents, owing to its thickness, the required mechanical resistance, and it is theoretically possible to utilize the envelope as a frame for tensioning the wires, provided that each wire is sealed at its ends to a sufficient portion of the thickness of the envelope wall.

In practice, manufacturing difiiculties arise because.

metal-to-glass welds are very diflicult to make whenever complete air-tightness is required and because such welds require the use of special metal alloys compatible with glass in regard of the thermal expansion coefficient. Moreover, wires, made from these alloys often are unable to withstand the high tension required for the aforementioned conditions.

The present invention has as object to eliminate these drawbacks and to obtain cathode ray tubes provided with large size grids, made up of stretched wire arrays, that satisfy the conditions mentioned above, while considerably simplifying the manufacturing.

-In accordance with the invention there is provided a cathode ray tube, comprising an air-tight glass envelope, consisting of two parts tightly sealed to each other, a fluorescent screen disposed in one of said portions, one or more electron guns disposed in the other portion for emitting electrons toward the screen, and a grid parallel to the screen, sealed between the two envelope portions and made up of metal wires extending to the outer surface of the envelope. The envelope surface zone which comprises the wire ends is coated with a layer of an electrically conducting substance, and the conducting layer itself is covered with a layer of sealing material adapted to ensure the air-tightness between the two portions of the envelope.

These and other features and advantages of the invention will become more apparent from the following description in connection with the accompanying drawings in which:

FIG. 1 represents schematically and in partial crosssection a color television picture tube of conventional shape, comprising a focusing grid made up of stretched, parallel metal wires;

FIG. 2 shows, in a partial, axial cross-section and on a large scale, an embodiment of the invention;

FIG. 3 represents the assembly of FIG. 2 in a stripped view; and

FIG. 4 shows in cross-section a plane face tube.

Referring to FIG. 1, there is shown a color television picture tube having a focusing and post-acceleration grid. The tube comprises an evacuated envelope composed of a flared or conically terminated rear portion 1 and a frontal portion or face 2 welded to each other along a sealing line 3 which may be of any form depending on the geometry of the front face, for example, circular or substantially rectangular.

The surface of the front face may be spherical, cylindrical or of any other suitable form. Adjacent the front face 2 are arranged, on the one hand, a luminescent screen 5, maintained in position by supports 5', and on the other hand, a plane array of parallel wires 4, forming a focusing grid. Screen 5 comprises triads of strips of luminescent substances of the fundamental colors chosen, for example, red, green and blue. The color strips and the grid wires are arranged perpendicularly to the plane of FIG. 1. It is known that the luminescent strips may also be deposited directly on the inner surface of the front face 2 of the tube.

In opera-tion, the grid and the screen are set at potentials Vg and Ve, respectively, by means of lead-in conduc tors provided in the envelope (not shown).

This type of tube is well known in the art and is shown only as an example to which the invention may be applied.

FIG. 2 shows in detail the way in which the wires of grid 4 are fixed by sealing between the front portion 2 and the cone 1 of the tube. It may be seen in this figure that each wire 4 is pinched between the rectified annular edges 1a and 2a of the cone and the front face, respectively, the sealing being made in the entire thickness of the wall by means of a ceramised glass cement. A portion of the seal is shown stripped in order to indicate the position of the wires 4 between the cone and the front face 2 of the tube. The wires are cut at 4a, that is flush with the seal, and a conducting layer 7 is applied on the entire periphery of the welding line and consequently on the cross-sections of the cut wires. The conducting layer 7 may be, for example, a conducting paint or a mixture of colloidal silver and ceramisable glass. By ceramisable glass is meant a kind of glass, such as the one known under the trade-mark Pyroceram, which becomes crystalline and acquires properties of ceramics after heating at a certain temperature for a determined time. An extension 7a of the conducting layer is connected to a contact or terminal 8, provided on the wall of the frontal tube portion.

The air tightness of the seal is achieved, in accordance with the invention, by an annular layer 9 of ceramisable glass, deposited over the conducting layer 7; this sealing layer is itself locked at the moment of application by a ribbon .10 made, for example, from woven glass fibres, which may without any inconvenience be left permanently on the tube.

FIG. 3, clearly shows the reciprocal positions of the different elements specified above, and particularly: the wires 4, seen here in cross-section, between the edges of cone 1 and the front face 2; the ceramisable glass material of seal 6; the conducting layer 7 with its extension 7a, connected to the contact or terminal 8; the sealing layer 9 of ceramisable glass which covers externally the conducting layer 7 and ensures the air-tightness of the seal, and finally, the loop of glass fibres 10.

An embodiment of the invention will now be described, by way of an illustrative example.

By means of any appropriate mechanical device, known per se, the grid is maintained in the form of a flat array of parallel wires, stretched to the desired tension.

A paste of ceramisable glass is applied to the sealing surfaces 1a and 2a, suitably rectified, of the cone and the front face, respectively. The cone and the front face are then brought together on either side of the wire array so that their edges clamp the wire array with a certain pressure to ensure good contact. Then the whole assembly is placed in an oven at a temperature and for a duration according to the type of ceramisable glass used, for example, one hour at 440 C. This thermal treatment removes the binder and carrier of the ceramisable glass paste: the ceramisable glass then acquires a solid vitrocrystalline state and forms a cement which firmly retains the wires and at the same time ensures the mutual seal of the cone and the front part of the tube.

After removing any possible burrs of the sealing material, the wires are cut flush with the outer wall, and the sealing line and wire extremities are coated with a paste mlade from a mixture of ceramisable glass and colloidal s1 ver.

This paste is also used for making the junction 7a to the terminal 8. Then the sealing layer 9, which may consist of the same ceramisable glass as the seal 6, is applied so as to cover completely the conducting layer 7. In order to retain the sealing layer 9 in position before its hardening, a glass fiber loop 10 is applied thereto. The whole assembly may then be submitted directly to the pumping process required for any electron tube to be evacuated. The heating, to which the tube is always submitted during the pumping process, may be utilized for baking-out the ceramisable glass of the layers 7 and 9. The heating may last, for example, 2 hours at a temperature of 420 C.

It is seen that one of the advantages provided by the invention lies in the simplification of the manufacture which requires here only one bake-out in addition to the one which is always necessary during the pumping process.

Moreover, the fastening of the tightened wires and the air tight sealing of the two parts of the envelope are ensured by two distinct elements.

Another advantage of the invention lies in the fact that, owing to its special properties, the ceramisable glass is capable to clamp the metal grid wires without resorting to the bonds which characterize a glass-metal weld.

On the other hand it is seen that in a tube in accordance with the invention the air-tightness is not due to the sealing of the grid wires in the glass of the envelope; therefore these wires may be made from non-magnetic stainless steel, having a thermal expansion coefficient much higher than that of the envelope glass. Hence, since the sealing is carried out when hot, the tension of the wires increases after cooling and thus the quality of the pictures is improved, as said above.

FIG. 4 shows in cross-section a picture tube in accordance With the invention, wherein the face plate has a plane inner surface.

The advantages of a tube whose face plate has a plane inner surface are well known; these advantages are obvious when the face plate carries the luminescent screen which includes the luminescent substances (phosphors).

However, a plane face plate increases the risk of implosion so that for avoiding prohibitive glass thicknesses tubes with a convex face plate (cylindrical or spherical) have generally been adopted.

But with a tube having a convex face plate the fluorescent screen and the grid cannot be accurately plane, or alternatively it is necessary to accept the drawbacks inherent in an independent screen, fixed in the bulb.

Now, applicant has verified experimentally that a grid structure, mounted as described hereinabove, increases considerably the implosion resistance of the bulb and allows to use a face plate with a reduced thickness.

The tube shown in FIG. 4 is a color television picture tube having three electron guns, a focusing grid, parallel luminescent strips, and a plane face plate. Shown in the figure are only those elements of the tube which are necessary for the understanding of the invention.

The front portion 11 of the tube comprises a plane face plate 14 and a rim or skirt 15. The grid 13 is directly welded between the skirt 15 and the cone 12.

The fluorescent screen 16, consisting of parallel luminescent strips reproducing the three primary colors (red, blue, green), deposited on the inner surface of face plate 14, is first coated with an aluminium film 17 and then with a layer 18 of porous graphite.

The screen is connected, through a conducting paint layer 111, to a terminal 19 intended to receive the screen potential Ve.

A groove 110, whose utility will be seen later, follows the inner profile of the screen.

An electrode 112 is connected internally to the grid 13 which is set at a potential Vg by means of the output connection 113.

Three electron guns 114, 115 and 116 are used to produce three electron beams, associated respectively with the three color luminescent strips.

Grid 13 is a part of the electron-optical system whose other elements have not been shown, except electrode 112, referred to as potential equalization electrode, which improves the operation of the grid.

The layer 16 of the luminescent strips forming the screen is deposited directly on the plane inner surface of face plate 14, which surface has previously been rectified, for example, by grinding so as to achieve a planeness with in less than one tenth of a millimeter over the whole of this surface.

The layer 16 is formed by parallel, vertical, contiguous strips, for example, 0.27 mm. wide, ensuring an adequate definition of the tube. Three consecutive strips form a triad, each strip corresponding to one of the three primary colors.

Owing to the fact that the inner surface of face plate 14 is perfectly plane, and making use of the groove on the outline of the screen, the luminescent strips may be deposited by any process suitable for a plane screen, in particular the known process of printing by contact a plane line array, utilizing sun exposure of a bichromatic colloid. The groove 110 permits to shift the line array for successively obtaining the strips of the three colors.

The grid 13 has the shape of a flat array of wires about 0.1 mm. in diameter (for example, of non-magnetic stainless steel), parallel to each other and to the layer 16, the

planeness of the array and the parallelism of the wires being achieved with great accuracy.

It should be noted here that one grid wire is associated with each triad of the screen.

Experiments have shown that an array of wires, stretched across the tube at the junction between the frontal tube portion 11 and the cone 12 provides an important strengthening of the tube for withstanding the outer pressure. This strengthening is probably due to a compensation of the tensile stresses on the tube wall in the break-up area constituted by the junction by the compressive stresses transmitted from the Wires 4 by the seal 6. This strengthening generally permits to reduce the thickness of the bulb by 25 to 33% with respect to the prior art.

Thus, for example, a thin glass bulb able to withstand only a pressure of the order of 2 kg./cm. without a wire grid, can withstand a pressure of 3 kg./cm. when provided with such a grid.

The invention permits of course to reduce the thickness of bulbs having convex as well as plane face plates.

Experimental research work, carried out by the applicant, has permitted to verify that a cathode ray tube having a glass envelope with a plane, substantially rectangular face plate, and provided with a grid structure such as described, fulfills the usual security conditions as soon as the thickness of the face plate is substantially equal to or slightly smaller than of the diagonal of the face plate.

For tubes with a plane face plate of circular shape, the thickness may be as low as of the diameter.

As illustrative examples, cathode ray tubes manufactured with plane, substantially rectangular face plates, have the required strength with the following dimen- Thickness of glass Diagonal of face plate: mm. 30 cm. 11 49 cm. 17 59 cm. 20

What is claimed is:

1. A cathode ray tube comprising, in combination:

(a) a glass envelope enclosing a cavity and having (1) a first part defining one portion of said cavity,

(2) a second part defining another portion of said cavity, said parts having respective annular edges;

(b) a fluorescent screen in said one portion;

(c) an electron gun positioned in said other portion to emit electrons toward said screen;

(d) a grid of parallel metal wires extending in said cavity in a plane substantially parallel to said screen and interposed between said gun and said screen, said wires essentially consisting of non-magnetic stainless steel having a coefiicient of thermal expansion substantially higher than the coefiicient of thermal expansion of the glass in said envelope;

(e) tension-maintaining means for maintaining said wires under tension and for transmiting the tension of the same to said envelope, said tension-maintaining means including a first annular body of vitreous sealing material interposed between said edges to connect the same, each of said wires having two end portions, each end portion being embedded in said body, extending through the same outward of said cavity, and having a transverse end face free from said body;

(f) a layer of conductive material superimposed on said end faces outside said cavity in conductive contact with said wires; and

(g) a second annular body of sealing material covering said conductive layer and connecting said first part to said second part in airtight engagement.

2. A tube as set forth in claim 1, further comprising a loop of glass fibers enveloping said second annular body.

3. A tube as set forth in claim 1, wherein said first part of said envelope has a plane face plate of circular shape substantially parallel to said screen, the thickness of said face plate being approximately of the diameter of said face plate.

4. A tube as set forth in claim 1, wherein said first part of said envelope has a plane substantially rectangular face plate substantially parallel to said screen, the thickness of said face plate being less than of the diagonal thereof.

References Cited UNITED STATES PATENTS 2,813,213 11/1957 Cramer et al.

2,864,032 12/ 1958 Amdursky et al. 2,899,575 8/1959 Vincent 31364 3,102,212 8/1963 Schlesinger 313-85 X 3,260,397 7/1966 De Gier et al.

3,284,653 11/1966 De France et al. 313283 X 3,284,655 11/1966 Oess 313286 2,875,363 2/1959 Nunan.

3,271,093 9/1966 Meier 31619 OTHER REFERENCES Kohl, Materials and Techniques for Electron Tubes, Reinhold Publishing Corporation, New York; copyrighted 1960 (TK 6565. V3 K65 in Scientific Library); pages 119, 120, 187, 233 and 234 cited.

ROBERT SEGAL, Primary Examiner U.S. Cl. X.R. 313-92, 283 

